This invention relates to a supporting apparatus for supporting semiconductor wafers when the semiconductor wafers are subjected to the heat treatment, for example, and a semiconductor wafer processing method using the supporting apparatus.
High temperature processes in a semiconductor device manufacturing method such as oxidation, annealing and LPCVD (Low Pressure Chemical Vapor Deposition) are effected by use of a furnace type tool. At present, a vertical furnace is generally used. The vertical furnace includes a quartz tube and boat which are stood up vertically and can process 100 or more wafers within one batch. However, the vertical furnace requires a long process time. This is mainly because of its large batch size of one process. If the number of wafers processed at one time is large, a relatively long time is required in each of the processing steps. That is, it takes a long time to transfer the wafers between the cassettes and the boats and a long time is taken until the temperature in the furnace becomes constant after the boat having the wafers mounted thereon is loaded into the furnace. Further, it takes a long time to raise or lower the temperature in the furnace and a relatively long process time is required because of the low film growth rate for maintaining the uniformity. Even if it takes a long process time, the batch size of the vertical furnace is large, and therefore, the throughput is relatively higher than the other type of semiconductor manufacturing device. However, recently, when advanced devices having a larger number of complicated manufacturing steps are developed, it is strongly required to reduce the processing time in order to enhance the turn around time. Therefore, it is desired to develop an apparatus and process with high throughput and short process time.
A mini-batch furnace is a means for satisfying the above requirements. The process time of several steps can be automatically reduced by reducing the batch size. However, the actual process time cannot be easily reduced simply by reducing the batch size. The same process time is required in the same processing condition even when the mini-batch furnace is used. In order to solve this problem, it is required to attain a higher film formation rate according to a new process condition suitable for the apparatus.
A condition of high temperature, high pressure and high gas flow rate using a wide-pitch wafer boat is a preferable measure. In the new process condition, the control operation for maintaining the uniformity in the film thickness in the wafer is one of the important issues. That is, because of its high film formation rate, a subtle difference in the distribution of gas pressure, content and flow rate will sensitively affect the uniformity of the film thickness.
Now, a new process condition is explained by taking a case wherein a BPSG (Boro-Phospho-Silicate Glass) film is formed by use of an LPCVD (low pressure chemical vapor deposition) method as an example.
FIG. 9A shows a conventional boat. The boat 100 is formed of quartz, for example, and has four rods 101 to 104 for supporting wafers 106. Each of the rods 101 to 104 has a plurality of slits 105 for supporting the wafers 106.
FIG. 10 shows a contour map of the film thickness of a BPSG film in a typical wafer in a case where the BPSG film is formed based on a new process condition by use of a conventional boat. It is clearly seen from FIG. 10 that four thin-film areas are present on the edge portion of the wafer. As shown in FIG. 9B, thin-film portions 107 lie on the outer edge portion of the wafer 106 and correspond in position to the rods 101 to 104 of the boat 100. The reason why the film thickness of areas near the rods 101 to 104 is reduced is considered as follows.
(1) The rods shield radiation heat from a resistance heater and the temperature of the portions of the wafer lying near the rods will not be sufficiently raised.
(2) The boat rods interrupt the process gas flow and the amount of gas which is brought into contact with the wafer is reduced.
(3) The boat rods construct a heat conduction path and the temperatures of portions of the wafer which are set in contact with the rods are lowered.
In the cases (1) and (3), the wafer temperature is locally lowered and the film deposition rate is lowered. In the case (2), the amount of a reaction gas is locally reduced and the film deposition rate is lowered. Thus, the conventional boat has various problems and it is desired to improve the boat.